AbstractSynthesis of monomers is the key and most labor-consuming step in the development of highly selective membrane materials. Polyalkylenesiloxanes show promise for separation of vapors of organic components from gas mixtures. The paper considers two approaches to the synthesis of 1,1,3,3,5,5-hexamethyl-2-oxa-1,3,5-trisilacyclohexane, a monomer for preparing poly-bis(dimethylsilmethylene)dimethylsiloxane, a promising polymer material for gas-separation and pervaporation membranes. Modified synthesis procedures using both approaches, closure of the six-membered ring via formation of the Si–O–Si or Si–C bond, are suggested. Comparative analysis shows that, among organomagnesium cyclization methods, the one-step method in a diethyl ether or diethyl glycol dibutyl ether should be preferred. The suggested procedure allows reaching the monomer yield as high as 75–80% and more.